1. Field of the Invention
The present invention relates to the optical structure of an optical head for use in a magnetooptical information record/reproducing apparatus.
2. Related Background
For use in a magnetooptical record/reproducing apparatus capable of verifying a recorded signal simultaneously with overwriting by magnetic field modulation, U.S. Pat. No. 5,293,569 issued Mar. 8, 1994, discloses an optical system of the optical head as shown in FIG. 1. In this drawing, S-direction is defined as the vertical direction parallel to a junction plane 4d of a crystal-coupled prism 4, consisting of rectangular monoaxial crystal prism 4a, 4b and a rectangular glass prism 4c, and the P-direction is defined as a direction perpendicular to the S-direction and also to the light beam advancing direction.
A semiconductor laser 1 is so positioned to emit linearly polarized light with a polarizing direction inclined by an angle .alpha. with respect to the P-direction. The diverging light beam from the semiconductor laser 1 is converted by a collimating lens 2 into a parallel light beam 50 which enters the rectangular prism 4a from an end face thereof. The rectangular prism 4a, consisting of the monoaxial crystal, has its optical (crystal) axis in the P-direction (with respect to the light beam 50). Consequently, before and after the reflection on the junction plane 4d, the p-component of the parallel light beam 50 is subjected to the extraordinary refractive index n.sub.e and the ordinary refractive index n.sub.o while the s-component of said beam is subjected only to the ordinary refractive index n.sub.o as shown in FIG. 2, whereby the light beam 50 is split into two linearly polarized light beams 100a, 100b (hereinafter collectively represented as 100) which have mutually perpendicular polarized directions and have a mutual angle therebetween in the horizontal plane. The light quantity ratio tan.sup.2 .alpha. of the p- and s-components is determined by the angle .alpha. of the polarized direction of the semiconductor laser 1. The ratio becomes 7:1 when .alpha.=20.7.degree.. The light beams 100 are guided by a deflecting mirror 5 and an objective lens 6 and form small spots SP1, SP2 on a same track 8 of a magnetooptical disk 7 constituting the optical information recording medium. When the disk 7 is rotated in a direction from SP2 to SP1 (indicated by an arrow), the angle .alpha. is so selected that, during the overwriting operation, the spot SP2 is exposed to a writing operation and the spot SP1 is exposed to a reading operation. In the overwriting operation, a magnetic field variable magnet 9 modulates the magnetic field according to the recording signal, thereby effecting recording at the position of the spot SP2, and the recorded signal is simultaneously read at the position of the succeeding spot SP1 to obtain a verifying signal. During the simple reproducing operation, the output power of the semiconductor laser is so regulated that the spot SP2 is exposed to a reading operation while the spot SP1 is exposed to a practically negligible low power, and the ordinary reproducing operation is conducted at the leading spot SP2. Such operations will not be explained as they are already described in detail in the aforementioned U.S. Patent Application.
The crystal-coupled prism 4 is provided with an evaporated half mirror (r.sub.p.sup.2 =0.5, r.sub.s.sup.2 =0.5) on the junction plane 4d between the prisms 4a, 4b.
The light 150 reflected by the magnetooptical disk 7 is guided to the crystal-coupled prism 4 through a light path substantially inverse to that toward the disk. The prism 4b of monoaxial crystal has its optical axis inclined by 45.degree. to the P- and S-directions. Consequently, the light beams entering the prism 4b through the junction plane 4d are subjected to the ordinary refractive index n.sub.o and the extraordinary refractive index n.sub.e, respectively at angles of 45.degree., whereby the light beam 150a is split into beams 200a, 200b while the light beam 150b is split into beams 200c, 200d. These emerging light beams 200a, 200b, 200c, 200d enter, through an imaging lens 10, respectively, four photosensor elements 11a, 11b, 11c, 11d of a photosensor 11. A servo signal and an RF signal can be obtained from the outputs of the photosensor elements 11a through 11d. For example, in case of overwriting, an RF signal for verifying can be obtained by (output of 11a-output of 11b), and, in the case of a simple reproduction operation, a magnetooptical signal as an RF reproduction signal can be obtained by (output of 11c-output of 11d).
However, in the incident optical system (forward path), the angle between the light beams 100a, 100b becomes as large as about 0.5.degree. when the rectangular prism 4a, 4b is made for example of rock crystals, and such light beams may form unsatisfactory spots if directly guided to the objective lens 6. For this reason it is conceived, as shown in FIG. 3, to insert a beam cross section shaping prism 12 along the direction of arrangement of the beams 100a, 100b. If the prism has a shaping ratio M on the beam cross section, the angle between the light beams proceeding toward the objective lens 6 becomes 1/M, whereby satisfactory image quality can be assured.
In such conventional configuration, if the junction plane 4d of the crystal-coupled prism 4 does not have a polarizing property, an intensity ratio of 7:1 between the spots SP2 and SP1 corresponds to tan.sup.2 .alpha.=1/7 or .alpha.=20.7.degree.. In the absence of the shaping prism, the two spots SP1, SP2 are made oval in shape because the far field of the light beam emitted from the semiconductor laser 1 is not rotationally symmetrical. Consequently, if such spots are positioned on a same track of the magnetooptical disk 7, the longer axis of the oval shape will have an angle of 20.7.degree. with respect to the longitudinal or perpendicular direction of the track, and such situation is undersirable for high-density information recording in the longitudinal direction of the track. This drawback still remains even when the shaping of beam cross section is applied, unless completely circular spots are obtained by complete shaping. Also, the narrower field direction of the light beam from the semiconductor laser 1 has an angle of 20.7.degree. with respect to the beam expanding direction in the cross section shaping. This fact, that the direction narrower in the far field of the light beam emitted from the semiconductor laser does not coincide with the expanding direction in beam cross section shaping, leads to another drawback of inefficient utilization of the light beam from the semiconductor laser 1.